As seen in the last chapter of MongoDB relationships, to implement a normalized database structure in MongoDB, we use the concept of Referenced Relationships also referred to as Manual References in which we manually store the referenced document”s id inside other document. However, in cases where a document contains references from different collections, we can use MongoDB DBRefs.

DBRefs vs Manual References

As an example scenario, where we would use DBRefs instead of manual references, consider a database where we are storing different types of addresses (home, office, mailing, etc.) in different collections (address_home, address_office, address_mailing, etc). Now, when a user collection”s document references an address, it also needs to specify which collection to look into based on the address type. In such scenarios where a document references documents from many collections, we should use DBRefs.

Using DBRefs

There are three fields in DBRefs −

• $ref − This field specifies the collection of the referenced document

• $id − This field specifies the _id field of the referenced document

• $db − This is an optional field and contains the name of the database in which the referenced document lies

Consider a sample user document having DBRef field address as shown in the code snippet −

{
   "_id":ObjectId("53402597d852426020000002"),
   "address": {
   "$ref": "address_home",
   "$id": ObjectId("534009e4d852427820000002"),
   "$db": "tutorialspoint"},
   "contact": "987654321",
   "dob": "01-01-1991",
   "name": "Tom Benzamin"
}

The address DBRef field here specifies that the referenced address document lies in address_home collection under tutorialspoint database and has an id of 534009e4d852427820000002.

The following code dynamically looks in the collection specified by $ref parameter (address_home in our case) for a document with id as specified by $id parameter in DBRef.

>var user = db.users.findOne({"name":"Tom Benzamin"})
>var dbRef = user.address
>db[dbRef.$ref].findOne({"_id":(dbRef.$id)})

The above code returns the following address document present in address_home collection −

{
   "_id" : ObjectId("534009e4d852427820000002"),
   "building" : "22 A, Indiana Apt",
   "pincode" : 123456,
   "city" : "Los Angeles",
   "state" : "California"
}